1. Field of the Invention
The invention concerns polyamides which are modified with dimer diol and dimer diol-containing hydroxyl-terminated polyesters. In comparison with the unmodified polyamides, the modified polyamides according to the invention have increased flexibility and viscosity even at low temperatures. No change in mechanical properties owing to loss or migration of softener on contact with media or under the influence of heat occurs due to the inherent softening. The flexible polyamides compared with other diol-containing polyamides show excellent resistance to hydrolysis. The invention further concerns a corresponding molding composition and use thereof.
2. Description of the Prior Art
For making polyamides flexible, numerous starting points are described in the literature such as the inclusion of additional polyamide-forming monomers in polyamide homopolymers, which improve the flexibility and low-temperature behavior.
Another possibility is afforded by incorporating polyamide-compatible softeners into polyamide molding compositions. Here sulphonamides and hydroxybenzoates which, like the polyamides themselves, are capable of forming hydrogen bridge bonds and reduce the glass transition temperature of the polyamides, are used in particular. Flexible polyamides are also obtained by compounding with at least one further highly flexible blending component, partly in combination with low molecular softeners. In EP 0 233 428 polyamide is processed with a polyetheramide block polymer and a thermoplastic elastomer from the group MBS, ABS, EPDM, NBR into a flexible formulation. DE 3 545 033 describes making polyamide flexible by the use of a combination of thermoplastic polyurethane elastomers and maleic acid-grafted polyolefins to make polyamide flexible.
N-alkylation of polyamides also has a flexible effect. The resulting decrease in hydrogen bridge bonds affects the properties of polyamide greatly. The polymer melting point drops, the solubility is increased and extended to a plurality of solvents and the polymer assumes rubber-like properties, depending on the degree of substitution.
By incorporating flexible polyester, polyether or polyolefin segments into polyamide block polymers, the result is likewise flexible polymers. In US document U.S. Pat. No. 5,574,128, both polyether diols and polyether diamines are described as soft segments in combination with polyamide blocks.
Similarly to the polyether esteramides, WO-8 502 852 describes flexible polyester amides with polycaprolactone segments in the main chain. Statistical polyester amides with a low-molecular diol component are described in EP 0 069 475.
According to this there are numerous possible ways of making homopolyamides or copolyamides flexible and giving them good impact strength even at low temperatures. Suitable low-molecular softeners can render substances flexible very effectively but without simultaneously ensuring very low impact strength at low temperatures, so that frequently compounding with impact strength modifiers is necessary. Furthermore softened polyamides have the disadvantage that the softener migrates into adjoining media or to the surface of work pieces and so the polymeric material becomes more and more brittle. Migration of the softener into adjoining media or to the surface of work pieces migrates and so the polymeric material becomes more and more brittle.
Polymer mixtures as a rule require a higher concentration of a component to render them flexible, so that similar moduli as with softened polyamides are obtained. But by this means frequently the properties typical of polyamides are lost.